1. Field of the Invention
The present invention relates generally to volume holography, and in particular, to a volume-holographic optical imaging instrument having the capability to return three-dimensional spatial as well as spectral information.
2. Description of the Related Art
Classical imaging systems process optical fields by using elements such as lenses, apertures and stops, and thin diffraction gratings. By placing several such elements in tandem, projections of very general objects (e.g., containing three-dimensional (3D) spatial as well as spectral information) may be captured. Such general objects are referred to as four dimensional (4D) objects. The projections of the 4D objects by the imaging system are two-dimensional (2D) or lower. Accordingly, to span the entire 3D or 4D space, scanning is needed. However, such scanning is a very time consuming process. Alternatively, instead of (or in combination with) scanning, various other prior art methods may be utilized (e.g., coherence imaging). However, such alternative methods may have a limited dynamic range or other disadvantages. To better understand these disadvantages, a description of prior art volume holography, imaging systems, and scanning mechanisms is useful.
Volume holography has been predominantly considered as a high-density data storage technology. With volume holography, the volume of the recording medium is utilized for storage instead of only utilizing the surface area (such as with compact discs [CDs] and/or digital video discs [DVDs]). Traditionally, when a laser is fired, a beam splitter is utilized to create two beams. One beam, referred to as the object or signal beam/wavefront travels through a spatial light modulator (SLM) that shows pages of raw binary data as clear and dark boxes. The information from the page of binary code is carried by the signal beam to a light-sensitive lithium-niobate crystal (or any other holographic materials such as a photopolymer in place of the crystal). The second beam (produced by the beam splitter), called the reference beam, proceeds through a separate path to the crystal. When the two beams meet, the interference pattern that is created stores the data carried by the signal beam in a specific area in the crystal as a hologram (also referred to as a holographic grating).
Depending on the angle of the reference beam used to store the data, various pages of data may be stored in the same area of the crystal. To retrieve data stored in the crystal, the reference beam is projected into the crystal at exactly the same angle at which it entered to store that page of data. If the reference beam is not projected at exactly the same angle, the page retrieval may fail. The beam is diffracted by the crystal thereby allowing the recreation of the page that was stored at the particular location. The recreated page may then be projected onto a charge-coupled device (e.g., CCD camera), that may interpret and forward the data to a computer.
Thus, as described above, a complex data-encoded signal wavefront is recorded inside a media as sophisticated holographic gratings by interference with a selective coherent reference beam. The signal wavefront is recovered later by reading out with the same corresponding reference beam.
Bragg's law determines that the diffracted light intensity is significant only when the diffracted light is spatially coherent and constructively in phase. Bragg's law is often used to explain the interference pattern of beams scattered by crystals. Due to the highly spatial and wavelength Bragg selectivity of a crystal, a large number of holograms can be stored and read out selectively in the same volume. Accordingly, there is a potential for one bit per wavelength cube data storage volume density and intrinsic parallelism of data accessing up to Mbytes per hologram.
The above-described properties also make a volume hologram a powerful tool for optical information processing. For example, a complex signal wavefront may be extracted and processed by one or multiple holograms as a color and spatial filter in confocal microscopes (see e.g., G. Barbastathis, M. Balberg, and D. J. Brady, “Confocal microscopy with a volume holographic filter,” Opt. Lett., vol. 24, no. 12, pp. 811–813, 1999 [which is incorporated by reference herein]). In another example, an element may be directly imaged for 3-D spatial and color information (see e.g., G. Barbastathis and D. J. Brady, “Multidimentional tomographic imaging using volume holography,” Proc. IEEE, vol. 87, no. 12, pp. 2098–2120, 1999; and G. G. Yang, H. S. Chen, and E. N. Leith, “Volume reflection holographic confocal imaging,” Appl. Opt., vol. 39, no. 23, pp. 4076–4079, 2000 [which articles are incorporated by reference herein]).
Optical information processing may be different from a data storage application where information is recorded inside the medium as complex holographic gratings. For imaging applications, simple pre-designed strong volume holograms may be recorded to process information from unknown complex incident wavefronts. The extremely spatial and color selectivity of Bragg matching in volume holograms makes it possible to selectively extract specific information from the input, and project them into one or multiple detectors.
As described above, prior art methods require scanning to span the entire 3D or 4D space. However, there are many limitations to such scanning.
A classical confocal microscope may be used to scan the 3D or 4D space. Confocal microscopes and their use are more fully illustrated in M. Minsky, “Microscopy apparatus,” U.S. Pat. No. 3,013,467 (Dec. 19, 1961); T. Wilson, Con focal Microscopy (Academic, San Diego, Calif., 1990); and J. K. Stevens, L. R. Mills, and J. E. Trogadis, eds., Three-Dimensional Confocasl Microscopy: Volume Investigation of Biological Systems (Academic, San Diego, Calif., 1994), which are incorporated by reference herein. Confocal microscopes may use a combination of objective-collector lenses and a pinhole to capture information about a single point in the object and acquires a zero-dimensional projection at every measurement. Scanning along three dimensions is needed to acquire the 3D spatial structure of the object. By providing spectral scanning means (e.g., a monochromator or a scanning Fabry-Perot interferometer), one can also acquire spectral information. However, such scanning is a very time-consuming procedure.
Another method for capturing spatial information is that of optical coherence tomography which only requires 3D scanning (see e.g., D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hen, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, Science 254, 1178 (1991), which is incorporated by reference herein). In such optical coherence tomography, spectral information may be recovered digitally from the phase of the correlation function of the optical beam (see e.g., U. Morgner, W. Drexler, F. X. Kartner, X. D. Li, C. Pitris, E. P. Ippen, and J. G. Fujimoto, Opt. Lett. 25, 111 (2000) which is incorporated by reference herein).
Coherence imaging may also be used. However, coherence imaging returns 2D projections in the Fourier (k) space at the expense of dynamic range (see e.g., W. H. Carter and E. Wolf, Opt. Acts 28, 227 (1981); K. Itoh and Y. Ohtsuka, J. Opt. Soc. Am. A 3, 94 (1986); J. Rosen and A. Yariv, J. Opt. Soc. Am. A 13, 2091 (1996); and D. L. Marks, R. A. Stack, D. J. Brady, D. C. Munson, Jr., and R. B. Brady, Science 284, 2164 (1999), which are incorporated by reference herein).
Accordingly, as described above, the prior art fails to provide a method, apparatus, or article of manufacture with the capability to quickly acquire spatial and spectral information simultaneously (in a single measurement). Such a failure forces 3D and 4D imaging to be dependent on the scanning speed of the mechanism used.